A passive optical network (PON) comprises an optical line terminal (OLT) connected to multiple optical network units (ONUs) in a point-to-multi-point network. New standards have been developed to define different types of PONs, each of which serves a different purpose. For example, the various PON types known in the related art include a Broadband PON (BPON), an Ethernet PON (EPON), a Gigabit PON (GPON), and others.
An exemplary diagram of a typical PON 100 is schematically shown in FIG. 1. The PON 100 includes M ONUs 120-1 through 120-M coupled to an OLT 130 via a passive optical splitter 140. Traffic data transmission may be achieved using GEM fragments or ATM cells over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, downstream transmission from the OLT 130 is broadcast to all the ONUs 120. Each ONU 120 filters its respective data according to, for example, pre-assigned labels.
The OLT 130 includes a transmitter (not shown) for transmitting downstream data to the ONUs 120 and a receiver (not shown) for receiving upstream burst data sent to OLT 130 from ONUs 120. The OLT 130 broadcasts data to the ONUs 120 along a common channel so that all the ONUs 120 receive the same data. Since all ONUs function in like manner, they will be collectively referred to by the reference numeral 120 in the following description unless reference is made to a specific ONU. Each of ONUs 120 includes a transmitter (not shown) for transmitting respective data to the OLT 130 during different time slots allocated by the OLT 130. To this end, the OLT 130 must allocate bandwidth to the ONUs 120 and specify when, during a complete cycle in which upstream data is sent from the ONUs 120 to the OLT 130, each ONU may transmit. That is, the OLT 130 is aware of the exact arrival time of data and the identity of a transmitting ONU 120.
The optical receiver at the OLT 130 must have the ability to cope with variation in the received optical power from burst to burst, since the path loss between each ONU 120 and the OLT 130 is different. Therefore, the OLT's 130 receiver requires a large dynamic range and should be able to set the threshold that discriminates low logic levels (“0” bits) from high logic levels (“1” bits) as quickly as possible. Alternatively, a transmitter of each ONU 120 may implement a power-leveling process, so as to allow adjustment of the output power at an ONU 120 according to the dynamic range of the OLT 130.
The power-leveling process is performed during initial set-up and normal operation modes of the PON. Generally, in the set-up mode an ONU 120 and the OLT 130 may be in one of the following operation states: serial number or ranging. In the former state, the OLT 130 tries to detect the serial number of an ONU 120. If the OLT 130 and the ONU 120 have not completed the serial number state protocol, due to a low power signal, the ONU 120 independently changes its optical power output until a successful detection of the serial number. In the ranging state, an ONU 120 transmits a ranging field in response to a ranging request. The OLT 130 measures the average optical power received from the ONU 120 and compares it to an OLT reception (RX) threshold. If the received power is above or below the RX threshold, the OLT 130 requests the specific ONU 120 to increase or decrease its output optical power.
During a normal operation mode, the OLT 130 periodically measures the average received optical power of a specific ONU 120 and compares it to a RX threshold. If the received power is above or below the RX threshold, the OLT 130 sends a message to the specific ONU 120 to decrease or increase respectively its optical power output. Typically, each transmitter in the ONUs 120 is configured with a fixed number of power states, each of which defines a different attenuation value. Upon receiving the message for changing the output power, the ONU transmitter changes its power state. The power-leveling process is described in detail in the respective PON standards, e.g., the ITU-T G.984.2 G.984.3 standards.
The power-leveling allows implementing optical receivers, at the OLTs, with narrower dynamic range, and thus reducing the cost of such receiver. However, on the other hand, such a solution requires complicated hardware in each ONU and a sophisticated power management protocol.
It would be therefore advantageous to provide a solution for power management control in PON systems that enables reliable burst-mode operation between the ONU and the OLT operating in distributed network environment.